Internet Engineering Task Force (IETF) S. Perreault, Ed.
Request for Comments: 6888 Viagenie
BCP: 127 I. Yamagata
Updates: 4787 S. Miyakawa
Category: Best Current Practice NTT Communications
ISSN: 2070-1721 A. Nakagawa
Japan Internet Exchange (JPIX)
H. Ashida
Cisco Systems
April 2013
Common Requirements for Carrier-Grade NATs (CGNs)
Abstract
This document defines common requirements for Carrier-Grade NATs
(CGNs). It updates RFC 4787.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6888.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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RFC 6888 CGN Requirements April 2013
This document builds upon previous works describing requirements for
generic NATs [RFC4787][RFC5382][RFC5508]. These documents, and their
updates if any, still apply in this context. What follows are
additional requirements, to be satisfied on top of previous ones.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Readers are expected to be familiar with "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP" [RFC4787] and the
terms defined there. The following additional term is used in this
document:
Carrier-Grade NAT (CGN): A NAT-based [RFC2663] logical function used
to share the same IPv4 address among several subscribers. A CGN
is not managed by the subscribers.
Note that the term "carrier-grade" has nothing to do with the
quality of the NAT; that is left to discretion of implementers.
Rather, it is to be understood as a topological qualifier: the
NAT is placed in an ISP's network and translates the traffic of
potentially many subscribers. Subscribers have limited or no
control over the CGN, whereas they typically have full control
over a NAT placed on their premises.
Note also that the CGN described in this document is IPv4-only.
IPv6 address translation is not considered.
However, the scenario in which the IPv4-only CGN logical
function is used may include IPv6 elements. For example, Dual-
Stack Lite (DS-Lite) [RFC6333] uses an IPv4-only CGN logical
function in a scenario making use of IPv6 encapsulation.
Therefore, this document would also apply to the CGN part of
DS-Lite.
Perreault, et al. Best Current Practice [Page 3]

RFC 6888 CGN Requirements April 2013
Figure 1 summarizes a common network topology in which a CGN
operates.
.
:
| Internet
............... | ...................
| ISP network
External pool: |
192.0.2.1/26 |
++------++ External realm
........... | CGN |...............
++------++ Internal realm
10.0.0.1 | |
| |
| | ISP network
............. | .. | ................
| | Customer premises
10.0.0.100 | | 10.0.0.101
++------++ ++------++
| CPE1 | | CPE2 | etc.
++------++ ++------++
(IP addresses are only for example purposes)
Figure 1: CGN Network Topology
Another possible topology is one for hotspots, where there is no
customer premise or customer premises equipment (CPE), but where a
CGN serves a bunch of customers who don't trust each other; hence,
fairness is an issue. One important difference with the previous
topology is the absence of a second layer of NAT. This, however, has
no impact on CGN requirements since they are driven by fairness and
robustness in the service provided to customers, which applies in
both cases.
3. Requirements for CGNs
What follows is a list of requirements for CGNs. They are in
addition to those found in other documents such as [RFC4787],
[RFC5382], and [RFC5508].
REQ-1: If a CGN forwards packets containing a given transport
protocol, then it MUST fulfill that transport protocol's
behavioral requirements. Current applicable documents are as
follows:
a. "NAT Behavioral Requirements for Unicast UDP" [RFC4787]
Perreault, et al. Best Current Practice [Page 4]

RFC 6888 CGN Requirements April 2013
b. "Network Address Translation (NAT) Behavioral Requirements for
TCP" [RFC5382]
c. "NAT Behavioral Requirements for ICMP" [RFC5508]
d. "Network Address Translation (NAT) Behavioral Requirements for
the Datagram Congestion Control Protocol (DCCP)" [RFC5597]
Any future NAT behavioral requirements documents for IPv4
transport protocols will impose additional requirements for CGNs
on top of those stated here.
Justification: It is crucial for CGNs to maximize the set of
applications that can function properly across them. The IETF has
documented the best current practices for UDP, TCP, ICMP, and
DCCP.
REQ-2: A CGN MUST have a default "IP address pooling" behavior of
"Paired" (as defined in Section 4.1 of [RFC4787]). A CGN MAY
provide a mechanism for administrators to change this behavior on
an application protocol basis.
* When multiple overlapping internal IP address ranges share the
same external IP address pool (e.g., DS-Lite [RFC6333]), the
"IP address pooling" behavior applies to mappings between
external IP addresses and internal subscribers rather than
between external and internal IP addresses.
Justification: This stronger form of REQ-2 from [RFC4787] is
justified by the stronger need for not breaking applications that
depend on the external address remaining constant.
Note that this requirement applies regardless of the transport
protocol. In other words, a CGN must use the same external IP
address mapping for all sessions associated with the same internal
IP address, be they TCP, UDP, ICMP, something else, or a mix of
different protocols.
The justification for allowing other behaviors is to allow the
administrator to save external addresses and ports for application
protocols that are known to work fine with other behaviors in
practice. However, the default behavior MUST be "Paired".
REQ-3: The CGN function SHOULD NOT have any limitations on the size
or the contiguity of the external address pool. In particular,
the CGN function MUST be configurable with contiguous or non-
contiguous external IPv4 address ranges.
Perreault, et al. Best Current Practice [Page 5]

RFC 6888 CGN Requirements April 2013
Justification: Given the increasing rarity of IPv4 addresses, it is
becoming harder for an operator to provide large contiguous
address pools to CGNs. Additionally, operational flexibility may
require non-contiguous address pools for reasons such as
differentiated services, routing management, etc.
The reason for having SHOULD instead of MUST is to account for
limitations imposed by available resources as well as constraints
imposed for security reasons.
REQ-4: A CGN MUST support limiting the number of external ports (or,
equivalently, "identifiers" for ICMP) that are assigned per
subscriber.
a. Per-subscriber limits MUST be configurable by the CGN
administrator.
b. Per-subscriber limits MAY be configurable independently per
transport protocol.
c. Additionally, it is RECOMMENDED that the CGN include
administrator-adjustable thresholds to prevent a single
subscriber from consuming excessive CPU resources from the CGN
(e.g., rate-limit the subscriber's creation of new mappings).
Justification: A CGN can be considered a network resource that is
shared by competing subscribers. Limiting the number of external
ports assigned to each subscriber mitigates the denial-of-service
(DoS) attack that a subscriber could launch against other
subscribers through the CGN in order to get a larger share of the
resource. It ensures fairness among subscribers. Limiting the
rate of allocation mitigates a similar attack where the CPU is the
resource being targeted instead of port numbers. However, this
requirement is not a MUST because it is very hard to explicitly
call out all CPU-consuming events.
REQ-5: A CGN SHOULD support limiting the amount of state memory
allocated per mapping and per subscriber. This may include
limiting the number of sessions, the number of filters, etc.,
depending on the NAT implementation.
a. Limits SHOULD be configurable by the CGN administrator.
b. Additionally, it SHOULD be possible to limit the rate at which
memory-consuming state elements are allocated.
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RFC 6888 CGN Requirements April 2013
Justification: A NAT needs to keep track of TCP sessions associated
with each mapping. This state consumes resources for which, in
the case of a CGN, subscribers may compete. It is necessary to
ensure that each subscriber has access to a fair share of the
CGN's resources. Limiting the rate of allocation is intended to
prevent CPU resource exhaustion. Item "B" is at the SHOULD level
to account for the fact that means other than rate limiting may be
used to attain the same goal.
REQ-6: It MUST be possible to administratively turn off translation
for specific destination addresses and/or ports.
Justification: It is common for a CGN administrator to provide
access for subscribers to servers installed in the ISP's network
in the external realm. When such a server is able to reach the
internal realm via normal routing (which is entirely controlled by
the ISP), translation is unneeded. In that case, the CGN may
forward packets without modification, thus acting like a plain
router. This may represent an important efficiency gain.
Figure 2 illustrates this use-case.
X1:x1 X1':x1' X2:x2
+---+from X1:x1 +---+from X1:x1 +---+
| C | to X2:x2 | | to X2:x2 | S |
| l |>>>>>>>>>>>>| C |>>>>>>>>>>>>>>| e |
| i | | G | | r |
| e |<<<<<<<<<<<<| N |<<<<<<<<<<<<<<| v |
| n |from X2:x2 | |from X2:x2 | e |
| t | to X1:x1 | | to X1:x1 | r |
+---+ +---+ +---+
Figure 2: CGN Pass-Through
REQ-7: It is RECOMMENDED that a CGN use an "endpoint-independent
filtering" behavior (as defined in Section 5 of [RFC4787]). If it
is known that "Address-Dependent Filtering" does not cause the
application-layer protocol to break (how to determine this is out
of scope for this document), then it MAY be used instead.
Justification: This is a stronger form of REQ-8 from [RFC4787].
This is based on the observation that some games and peer-to-peer
applications require EIF for the NAT traversal to work. In the
context of a CGN, it is important to minimize application
breakage.
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RFC 6888 CGN Requirements April 2013
REQ-8: Once an external port is deallocated, it SHOULD NOT be
reallocated to a new mapping until at least 120 seconds have
passed, with the exceptions being:
a. If the CGN tracks TCP sessions (e.g., with a state machine, as
in Section 3.5.2.2 of [RFC6146]), TCP ports MAY be reused
immediately.
b. If external ports are statically assigned to internal
addresses (e.g., address X with port range 1000-1999 is
assigned to subscriber A, 2000-2999 to subscriber B, etc.),
and the assignment remains constant across state loss, then
ports MAY be reused immediately.
c. If the allocated external ports used address-dependent or
address-and-port-dependent filtering before state loss, they
MAY be reused immediately.
The length of time and the maximum number of ports in this state
MUST be configurable by the CGN administrator.
Justification: This is necessary in order to prevent collisions
between old and new mappings and sessions. It ensures that all
established sessions are broken instead of redirected to a
different peer.
The exceptions are for cases where reusing a port immediately does
not create a possibility that packets would be redirected to the
wrong peer. One can imagine other exceptions where mapping
collisions are avoided, thus justifying the SHOULD level for this
requirement.
The 120 seconds value corresponds to the Maximum Segment Lifetime
(MSL) from [RFC0793].
Note that this requirement also applies to the case when a CGN
loses state (due to a crash, reboot, failover to a cold standby,
etc.). In that case, ports that were in use at the time of state
loss SHOULD NOT be reallocated until at least 120 seconds have
passed.
REQ-9: A CGN MUST implement a protocol giving subscribers explicit
control over NAT mappings. That protocol SHOULD be the Port
Control Protocol [RFC6887].
Justification: Allowing subscribers to manipulate the NAT state
table with PCP greatly increases the likelihood that applications
will function properly.
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RFC 6888 CGN Requirements April 2013
A study of PCP-less CGN impacts can be found in [NAT444]. Another
study considering the effects of PCP on a peer-to-peer file
sharing protocol can be found in [BITTORRENT].
REQ-10: CGN implementers SHOULD make their equipment manageable.
Standards-based management using standards such as "Definitions of
Managed Objects for NAT" [RFC4008] is RECOMMENDED.
Justification: It is anticipated that CGNs will be primarily
deployed in ISP networks where the need for management is
critical. This requirement is at the SHOULD level to account for
the fact that some CGN operators may not need management
functionality.
Note also that there are efforts within the IETF toward creating a
MIB tailored for CGNs (e.g., [NAT-MIB]).
REQ-11: When a CGN is unable to create a dynamic mapping due to
resource constraints or administrative restrictions (i.e.,
quotas):
a. it MUST drop the original packet;
b. it SHOULD send an ICMP Destination Unreachable message with
code 1 (Host Unreachable) to the sender;
c. it SHOULD send a notification (e.g., SNMP trap) towards a
management system (if configured to do so); and
d. it MUST NOT delete existing mappings in order to "make room"
for the new one. (This only applies to normal CGN behavior,
not to manual operator intervention.)
Justification: This is a slightly different form of REQ-8 from
[RFC5508]. Code 1 is preferred to code 13 because it is listed as
a "soft error" in [RFC1122], which is important because we don't
want TCP stacks to abort the connection attempt in this case. See
[RFC5461] for details on TCP's reaction to soft errors.
Sending ICMP errors and SNMP traps may be rate-limited for
security reasons, which is why requirements B and C are SHOULDs,
not MUSTs.
Applications generally handle connection establishment failure
better than established connection failure. This is why dropping
the packet initiating the new connection is preferred over
deleting existing mappings. See also the rationale in Section 6
of [RFC5508].
Perreault, et al. Best Current Practice [Page 9]

RFC 6888 CGN Requirements April 20134. Logging
It may be necessary for CGN administrators to be able to identify a
subscriber based on external IPv4 address, port, and timestamp in
order to deal with abuse. When multiple subscribers share a single
external address, the source address and port that are visible at the
destination host have been translated from the ones originated by the
subscriber.
In order to be able to do this, the CGN would need to log the
following information for each mapping created (this list is for
informational purposes only and does not constitute a requirement):
o transport protocol
o subscriber identifier (e.g., internal source address or tunnel
endpoint identifier)
o external source address
o external source port
o timestamp
By "subscriber identifier" we mean information that uniquely
identifies a subscriber. For example, in a traditional NAT scenario,
the internal source address would be sufficient. In the case of DS-
Lite, many subscribers share the same internal address and the
subscriber identifier is the tunnel endpoint identifier (i.e., the
B4's IPv6 address).
A disadvantage of logging mappings is that CGNs under heavy usage may
produce large amounts of logs, which may require large storage
volume.
REQ-12: A CGN SHOULD NOT log destination addresses or ports unless
required to do so for administrative reasons.
Justification: Destination logging at the CGN creates privacy
issues. Furthermore, readers should be aware of logging
recommendations for Internet-facing servers [RFC6302]. With
compliant servers, the destination address and port do not need to
be logged by the CGN. This can help reduce the amount of logging.
This requirement is at the SHOULD level to account for the fact
that there may be other reasons for logging destination addresses
or ports. One such reason might be that the remote server is not
following [RFC6302].
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RFC 6888 CGN Requirements April 20135. Port Allocation Scheme
A CGN's port allocation scheme is subject to three competing
requirements:
REQ-13: A CGN's port allocation scheme SHOULD maximize port
utilization.
Justification: External ports are one of the resources being shared
by a CGN. Efficient management of that resource directly impacts
the quality of a subscriber's Internet connection.
Some schemes are very efficient in their port utilization. In
that sense, they have good scaling properties (nothing is wasted).
Others will systematically waste ports.
REQ-14: A CGN's port allocation scheme SHOULD minimize log volume.
Justification: Huge log volumes can be problematic to CGN operators.
Some schemes create one log entry per mapping. Others allow
multiple mappings to generate a single log entry, which sometimes
can be expressed very compactly. With some schemes, the logging
frequency can approach that of DHCP servers.
REQ-15: A CGN's port allocation scheme SHOULD make it hard for
attackers to guess port numbers.
Justification: Easily guessed port numbers put subscribers at risk
of the attacks described in [RFC6056].
Some schemes provide very good security in that ports numbers are
not easily guessed. Others provide poor security to subscribers.
A CGN implementation's choice of port allocation scheme optimizes to
satisfy one requirement at the expense of another. Therefore, these
are soft requirements (SHOULD as opposed to MUST).
6. Deployment Considerations
Several issues are encountered when CGNs are used [RFC6269]. There
is current work in the IETF toward alleviating some of these issues.
For example, see [NAT-REVEAL].
Perreault, et al. Best Current Practice [Page 11]